A well known homogeneous catalyst is, for example, so-called Kaminsky catalyst. Use of this Kaminsky catalyst produces a polymer having an extremely high polymerization activity and a narrow molecular weight distribution.
Of the Kaminsky catalysts, ethylenebis(indenyl)-zirconium dichloride and ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride are known as transition metal compounds for preparing isotactic polyolefins, as described in Japanese Patent Laid-Open Publication No. 130314/1986. However, polyolefins prepared by the use of these catalysts generally have a low stereoregularity and a low molecular weight. As a process for preparing polyolefins of high stereoregularity and high molecular weight using these catalyst, there is a process in which the polymerization is conducted at a low temperature, but this process has a problem of low polymerization activity.
It is known that use of hafnium compounds in place of the zirconium compounds makes it possible to prepare a polymer having high molecular weight, as described in "Journal of Molecular Catalysis", 56 (1989), pp. 237-247, but this process also has a problem of low polymerization activity. Further, dimethylsilyl bissubstituted cyclopentadienyl zirconium dichloride is also known as described in Japanese Patent Laid-Open Publication No. 301704/1989 and "Polymer Preprints", Japan, vol. 39, No. 6, pp. 1,614-1,616 (1990), but this compound is not satisfactory in all of polymerization activity, and stereoregularity and molecular weight of polymers obtained.
In order to solve these problems, various proposals have been made. For example, Japanese Patent Laid-Open Publication 268307/1993 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and aluminoxane as a catalyst capable of preparing a high molecular polyolefin. ##STR2##
Further, EP 0 530 648 A1 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and aluminoxane. ##STR3## wherein A is a lower alkyl group.
However, the stereoregularity and the molecular weight of the polyolefin obtained by the use of these catalysts are not always satisfactorily, and the amount of inversely inserted units is still too large.
Moreover, a catalyst component (wherein, A is a phenyl group or naphthyl group in the aforementioned metallocene compound) is published from HOECHST AKTIENGESELLSCHAFT at 40 YEARS ZIEGLER CATALYST IN HONOR OF KARL ZIEGER AND WORKSHOP (Sep. 1-3, 1993).
Furthermore, EP 0 576 970 A1 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and an aluminoxane. ##STR4## wherein M.sup.1 is a transition metal atom, R.sup.1 and R.sup.2 are each a holagen atom, etc., R.sup.3 is an alkyl group of 1 to 10 carbon atoms, etc., R.sup.4 to R.sup.12 are each an alkyl group of 1 to 10 carbon atoms etc., R.sup.13 is a hydrocarbon group or a silicon containing group.
However, the stereoregularity of the polyolefin obtained by the use of these catalysts are not always satisfactorily, and the amount of inversely inserted units is still too large.
In the light of such prior arts as described above, the present inventors have found that polymerization activity of the catalyst component comprising the aforementioned transition metal compound is depending upon the kind of substituent on the indenyl group, and varied markedly in the stereoregularity and the amount of the inversely inserted units of the resulting polyolefin. Further, the inventors have also found that the transition metal compound having indenyl groups containing a specific substituent as a ligand is excellent olefin polymerization activity, and is capable of giving an olefin polymerization catalyst which provides an olefin polymer having high stereoregularity and low in the amount of inversely inserted units.
Propylene polymers, especially propylene homopolymers, have been applied to various uses such as industrial parts, containers, films and nonwoven fabrics, because of their excellent rigidity, surface hardness, heat resistance, glossiness and transparency.
However, the conventional propylene homopolymer is not always sufficient in transparency, impact resistance, etc. for some uses, and therefore the advent of a propylene polymer excellent in rigidity, heat resistance, surface hardness, glossiness, transparency and impact strength is desired.
Moreover, the physical properties of the copolymers of propylene and an .alpha.-olefin other than propylene vary depending on composition thereof, and hence the copolymers are generally distinguishable from each other bordering the monomer content derived from the .alpha.-olefin other than propylene of 5% by mol.
Propylene copolymers containing monomer units derived from .alpha.-olefin other than propylene in an amount of less than 5% by mol have been applied to various uses such as containers and packaging materials (e.g., films), because of their excellent rigidity, surface hardness, heat resistance, transparency and heat-sealing property. However, when the copolymer is used as a film, the resulting film is not always sufficient in transparency, heat-sealing property, anti-blocking property, anti-bleedout property and impact strength. Therefore, a propylene copolymer further improved in transparency, rigidity, surface hardness, heat resistance and heat-sealing property, and having excellent anti-blocking property, anti-bleedout property and impact strength is desired.
In contrast, propylene copolymers containing monomer units derived from .alpha.-olefin other than propylene in an amount of more than 5% by mol have been applied to various uses such as films, heat-sealing layers of laminated films, and modifiers for improving impact resistance and anti-heat-sealing property of thermoplastic resins, because of their excellent transparency, heat-sealing property at low temperature, environmental aging property and impact absorbing capacity. However, the conventional propylene copolymer is not always sufficient in transparency, heat-sealing properties at low temperature, anti-blocking properties, bleedout resistance, impact strength, etc. for some uses, and the modifiers therefrom are not always sufficient in effect of improving heat-sealing property at low temperature and impact strength. Therefore, there has been demanded a propylene copolymer further improved in transparency, environmental aging property and impact strength, and having excellent in effect of improving heat-sealing property at low temperature and impact strength.
In the light of such circumstances as described above, the present inventors have further studied, and as a result, they have found that a propylene homopolymer obtained by homopolymerization of propylene in the presence of an olefin polymerization catalyst containing a specific transition metal compound, and a propylene copolymer obtained by copolymerization of propylene and at least one kind of .alpha.-olefin selected from the group consisting of ethylene and .alpha.-olefins having 4 to 20 carbon atoms satisfy the above mentioned requisites.
A propylene/ethylene random copolymer containing a small amount of ethylene units is excellent in transparency, rigidity, surface hardness, heat resistance, and hence it is used for films, containers etc.
Heretofore, there is known some methods for preparation of the propylene/ethylene random copolymer containing a small amount of ethylene units, such as a method using a titanium catalyst system comprising a titanium compound and an organoaluminum compound and a method using a metallocene catalyst system comprising a metallocene compound (e.g., zirconocene and hafnocene) and an alkylaluminoxane or ionic compound.
However, the propylene/ethylene random copolymer obtained by using a titanium catalyst system is not always sufficient in heat-sealing property for some uses, and also insufficient in anti-blocking property, bleedout property and impact strength. On the other hand, the propylene/ethylene random copolymer obtained by using a metallocene catalyst system is not always sufficient in rigidity, surface hardness and heat resistance. Therefore, the advent of the propylene/ethylene random copolymer having advantages of the both, and excellent in balance of properties is demanded.
In the light of such circumstances as described above, the present inventors have further studied, and as a result, they have found that a propylene copolymer containing a specific amount of ethylene unit, having a high triad tacticity, as measured by .sup.13 C-NMR, of the propylene chain consisting of head-to-tail bonds, a specific proportion of inversely inserted propylene units and a specific intrinsic viscosity is excellent in transparency, rigidity, surface hardness, heat-sealing property, anti-blocking property, anti-bleedout property and impact strength.
Further, the propylene elastomer is excellent in impact absorbing capacity, heat resistance and heat-sealing property, it is singly used for films, and also is used for modifier for thermoplastic resin.
However, when the conventional propylene elastomer is singly used for films, the resulting films are not always sufficient in heat-sealing property, anti-blocking property and heat resistance. When the elastomer is used for modifier, the effect of improving impact strength is not always sufficient. Therefore, the advent of the propylene elastomer having excellent impact strength, and effective in improving heat resistance, transparency, heat-sealing property, anti-blocking resistance and impact resistance is demanded.
In the light of such circumstances as described above, the present inventors have further studied, and as a result, they have found that a propylene elastomer containing a specific amount of ethylene unit, having a high triad tacticity, as measured by .sup.13 C-NMR, of the propylene chain consisting of head-to-tail bonds, a specific proportion of inversely inserted propylene units and a specific intrinsic viscosity is excellent in above mentioned properties, and hence accomplished the present invention.